I had the same issues as I was buying from a German seller who spoke very little English.. It leads to a little anxiety and frustration when you've sent off your 'hard earned' and can't communicate properly with the seller when you're wondering where the unit is!
I'm not familiar with the process in an A3 but can't imagine it would be too different from what it involved in the A4.
Is the A6 RNS-E the correct unit for retrofitting in a first gen A3 (8L)? If not, you could potentially have to do some 'modifying' of the faceplate.
I'm sure someone here has done the job before so a search will probably be the best place to start!